553 research outputs found
HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep
Circumstellar shells around AGB stars are built over long periods of time
that may reach several million years. They may therefore be extended over large
sizes (~1 pc, possibly more), and different complementary tracers are needed to
describe their global properties. In the present work, we combined 21-cm HI and
CO rotational line data obtained on an oxygen-rich semi-regular variable, RX
Lep, to describe the global properties of its circumstellar environment. With
the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile
is parabolic and implies an expansion velocity of ~4.2 km/s and a mass-loss
rate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected with
the Nancay Radiotelescope on the star position and at several offset positions.
The linear shell size is relatively small, ~0.1 pc, but we detect a trail
extending southward to ~0.5 pc. The line profiles are approximately Gaussian
with an FWHM ~3.8 km/s and interpreted with a model developed for the detached
shell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproduced
by assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 years
duration, which has been slowed down by the external medium. The spatial offset
of the HI source is consistent with the northward direction of the proper
motion, lending support to the presence of a trail resulting from the motion of
the source through the ISM, as already suggested for Mira, RS Cnc, and other
sources detected in HI. The source was also observed in SiO (3 mm) and OH (18
cm), but not detected. The properties of the external parts of circumstellar
shells around AGB stars should be dominated by the interaction between stellar
outflows and external matter for oxygen-rich, as well as for carbon-rich,
sources, and the 21-cm HI line provides a very useful tracer of these regions.Comment: 15 pages, 9 figures, accepted for publication in A&
Inner disk clearing around the Herbig Ae star HD\,139614: Evidence for a planet-induced gap ?
Spatially resolving the inner dust cavity of the transitional disks is a key
to understanding the connection between planetary formation and disk dispersal.
The disk around the Herbig star HD 139614 is of particular interest since it
presents a pretransitional nature with an au-sized gap, in the dust, that was
spatially resolved by mid-IR interferometry. Using new NIR interferometric
observations, we aim to characterize the 0.1-10~au region of the HD~139614 disk
further and identify viable mechanisms for the inner disk clearing. We report
the first multiwavelength radiative transfer modeling of the interferometric
data acquired on HD~139614 with PIONIER, AMBER, and MIDI, complemented by
Herschel/PACS photometries. We confirm a gap structure in the um-sized dust,
extending from about 2.5 au to 6 au, and constrained the properties of the
inner dust component: e.g., a radially increasing surface density profile, and
a depletion of 10^3 relative to the outer disk. Since self-shadowing and
photoevaporation appears unlikely to be responsible for the au-sized gap of
HD~139614, we thus tested if dynamical clearing could be a viable mechanism
using hydrodynamical simulations to predict the gaseous disk structure. Indeed,
a narrow au-sized gap is expected when a single giant planet interacts with the
disk. Assuming that small dust grains are well coupled to the gas, we found
that a ~ 3~Mjup planet located at 4.5 au from the star could, in less than 1
Myr, reproduce most of the aspects of the dust surface density profile, while
no significant depletion in gas occurred in the inner disk, in contrast to the
dust. However, the dust-depleted inner disk could be explained by the expected
dust filtration by the gap and the efficient dust growth/fragmentation in the
inner disk regions. Our results support the hypothesis of a giant planet
opening a gap and shaping the inner region of the HD~139614 disk.Comment: Version accepted in A&A, with typos corrections in the tex
On the reliability of mass-loss-rate estimates for AGB stars
In the recent literature there has been some doubt as to the reliability of
CO multi-transitional line observations as a mass-loss-rate estimator for AGB
stars. Mass-loss rates for 10 intermediate- to high-mass-loss-rate AGB stars
are derived using a detailed non-LTE, non-local radiative transfer code based
on the Monte-Carlo method to model the CO radio line intensities. The
circumstellar envelopes are assumed to be spherically symmetric and formed by
constant mass-loss rates. The energy balance is solved self-consistently and
the effects of dust on the radiation field and thermal balance are included. An
independent estimate of the mass-loss rate is also obtained from the
combination of dust radiative transfer modelling with a dynamical model of the
gas and dust particles. We find that the CO radio line intensities and shapes
are successfully reproduced for the majority of our objects assuming a constant
mass-loss rate. Moreover, the CO line intensities are only weakly dependent on
the adopted micro-turbulent velocity, in contrast to recent claims in the
literature. The two methods used in the present work to derive mass-loss-rates
are consistent within a factor of ~3 for intermediate- to high-mass-loss-rate
objects, indicating that this is a lower limit to the uncertainty in present
mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss
rates from the dust/dynamical model are systematically higher than those from
the CO model for the carbon stars and vice versa for the M-type stars. This
could be ascribed to a discrepancy in the adopted CO/H_2-abundance ratio, but
we caution that the sample is small and systematic errors cannot be excluded.Comment: 18 pages, 17 figures, accepted for publication in A&
Detection of Vibrationally Excited CO in IRC+10216
Using the Submillimeter Array we have detected the J=3-2 and 2-1 rotational
transitions from within the first vibrationally excited state of CO toward the
extreme carbon star IRC+10216 (CW Leo). The emission remains spatially
unresolved with an angular resolution of ~2" and, given that the lines
originate from energy levels that are ~3100 K above the ground state, almost
certainly originates from a much smaller (~10^{14} cm) sized region close to
the stellar photosphere. Thermal excitation of the lines requires a gas density
of ~10^{9} cm^{-3}, about an order of magnitude higher than the expected gas
density based previous infrared observations and models of the inner dust shell
of IRC+10216.Comment: Accepted for publication in ApJ Letter
Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
Multiplicity is one of the most fundamental observable properties of massive
O-type stars and offers a promising way to discriminate between massive star
formation theories. Nevertheless, companions at separations between 1 and 100
mas remain mostly unknown due to intrinsic observational limitations. [...] The
Southern MAssive Stars at High angular resolution survey (SMASH+) was designed
to fill this gap by providing the first systematic interferometric survey of
Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162
O-type stars with NACO/SAM, respectively probing the separation ranges 1-45 and
30-250mas and brightness contrasts of Delta H < 4 and Delta H < 5. Taking
advantage of NACO's field-of-view, we further uniformly searched for visual
companions in an 8''-radius down to Delta H = 8. This paper describes the
observations and data analysis, reports the discovery of almost 200 new
companions in the separation range from 1mas to 8'' and presents the catalog of
detections, including the first resolved measurements of over a dozen known
long-period spectroscopic binaries.
Excluding known runaway stars for which no companions are detected, 96
objects in our main sample (DEC < 0 deg; H<7.5) were observed both with PIONIER
and NACO/SAM. The fraction of these stars with at least one resolved companion
within 200mas is 0.53. Accounting for known but unresolved spectroscopic or
eclipsing companions, the multiplicity fraction at separation < 8'' increases
to f_m = 0.91 +/- 0.03. The fraction of luminosity class V stars that have a
bound companion reaches 100% at 30mas while their average number of physically
connected companions within 8'' is f_c = 2.2 +/- 0.3. This demonstrates that
massive stars form nearly exclusively in multiple systems. Additionally, the
nine non-thermal (NT) radio emitters observed by SMASH+ are all resolved [...]Comment: 57 pages, 20 figures, 7 tables; accepted for publication in ApJ
A Review of AGB Mass Loss Imaging Techniques
Circumstellar imaging, across the electromagnetic spectrum, allows to derive
fundamental diagnostics for the physics of mass loss in the AGB phase. I review
the current status of the field, with particular emphasis on the techniques
that provide the strongest constraints for mass loss modeling efforts.Comment: accepted for publication in the Publications of the Astronomical
Society of Australi
Sculpting the disk around T Cha: an interferometric view
(Abridged) Circumstellar disks are believed to be the birthplace of planets
and are expected to dissipate on a timescale of a few Myr. The processes
responsible for the removal of the dust and gas will strongly modify the radial
distribution of the dust and consequently the SED. In particular, a young
planet will open a gap, resulting in an inner disk dominating the near-IR
emission and an outer disk emitting mostly in the far-IR. We analyze a full set
of data (including VLTI/Pionier, VLTI/Midi, and VLT/NaCo/Sam) to constrain the
structure of the transition disk around TCha. We used the Mcfost radiative
transfer code to simultaneously model the SED and the interferometric
observations. We find that the dust responsible for the emission in excess in
the near-IR must have a narrow temperature distribution with a maximum close to
the silicate sublimation temperature. This translates into a narrow inner dusty
disk (0.07-0.11 AU). We find that the outer disk starts at about 12 AU and is
partially resolved by the Pionier, Sam, and Midi instruments. We show that the
Sam closure phases, interpreted as the signature of a candidate companion, may
actually trace the asymmetry generated by forward scattering by dust grains in
the upper layers of the outer disk. These observations help constrain the
inclination and position angle of the outer disk. The presence of matter inside
the gap is difficult to assess with present-day observations. Our model
suggests the outer disk contaminates the interferometric signature of any
potential companion that could be responsible for the gap opening, and such a
companion still has to be unambiguously detected. We stress the difficulty to
observe point sources in bright massive disks, and the consequent need to
account for disk asymmetries (e.g. anisotropic scattering) in model-dependent
search for companions.Comment: Removed the word "first" in the abstract of the paper: "obtained with
the first 4-telescope combiner (VLTI/Pionier)
Entire large solutions for semilinear elliptic equations
We analyze the semilinear elliptic equation , in
, with a particular emphasis put on the qualitative
study of entire large solutions, that is, solutions such that
. Assuming that satisfies the
Keller-Osserman growth assumption and that decays at infinity in a
suitable sense, we prove the existence of entire large solutions. We then
discuss the more delicate questions of asymptotic behavior at infinity,
uniqueness and symmetry of solutions.Comment: Journal of Differential Equations 2012, 28 page
GG Tau: the fifth element
We aim at unveiling the observational imprint of physical mechanisms that
govern planetary formation in young, multiple systems. In particular, we
investigate the impact of tidal truncation on the inner circumstellar disks. We
observed the emblematic system GG Tau at high-angular resolution: a
hierarchical quadruple system composed of low-mass T Tauri binary stars
surrounded by a well-studied, massive circumbinary disk in Keplerian rotation.
We used the near-IR 4-telescope combiner PIONIER on the VLTI and
sparse-aperture-masking techniques on VLT/NaCo to probe this proto-planetary
system at sub-AU scales. We report the discovery of a significant closure-phase
signal in H and Ks bands that can be reproduced with an additional low-mass
companion orbiting GG Tau Ab, at a (projected) separation rho = 31.7 +/- 0.2mas
(4.4 au) and PA = 219.6 +/- 0.3deg. This finding offers a simple explanation
for several key questions in this system, including the missing-stellar-mass
problem and the asymmetry of continuum emission from the inner dust disks
observed at millimeter wavelengths. Composed of now five co-eval stars with
0.02 <= Mstar <= 0.7 Msun, the quintuple system GG Tau has become an ideal test
case to constrain stellar evolution models at young ages (few 10^6yr).Comment: 5pages, 3 figures, 1 appendix (online material
Atomic hydrogen in AGB circumstellar environments. A case study: X Her
We report the detection of the HI line at 21 cm from the circumstellar shell
around the AGB star X Her using the position-switching technique with the
Nancay Radio Telescope. At the star position the line shows 2 components: (i) a
broad one (FWHM ~ 13 km/s) centered at -72.2 km/s, and (ii) a narrow one (FWHM
\~ 4 km/s) centered at ~ -70.6 km/s. Our map shows that the source associated
to the broad component is asymmetric with material flowing preferentially
towards the North-East. This source extends to ~ 10 arcmin. (~ 0.4 pc) from the
star in that direction. On the other hand, the narrow component is detected
only at the star position and indicates material flowing away from the
observer. The total mass of atomic hydrogen is ~ 6.5 10^{-3} solar mass which,
within a factor 2, agrees with the estimate obtained from IRAS data at 60
microns.Comment: accepted for publication in MNRA
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